Fabrication of electrical units



March 26, 1963 w. DOELP, JR

FABRICATION OF ELECTRICAL UNITS Original Filed Sept. 20, 1957 6 6 2 mi MUnited States Patent sense;

1 Claim. (e1. 29-497 This invention relates to a method of fabricatingsemiconductor devices. The invention has to do particularly with theinformation of certain electrode means, as used for instance inso-called alloy junction power transistors. The present application is adivision of an earlier application, filed on September 20, 1957, underSerial No. 685,232, now Patent No. 2,916,604, and which pertains toapparatus for fabrication of the present type.

Much progress has been made in such fabrication, during recent years,but the invention has provided further improvement. It had been felt tobe desirable to accelerate certain processes, including notably that offorming and connecting collector electrodes, and at the same time toproduce transistors which comply more consistently with requirements.Under the most advanced techniques, heretofore available in this field,it Was still necessary either to accept the "formation of transistorswhich varied greatly as to some parameters, or to take relatively longperiods of time for certain types of electrode formation or connection,for instance by immersion heating or the like, or in some cases to copewith both types of problems. As .a result, the cost of finished, highquality transistors has still been somewhat substantial, althoughprogress had been made in eliminating some of the original costelements.

It is therefore among the general objects of the present invention toavoid complications, inconsistencies, costs, and losses of time, infabrication of the type referred to. A more specific object is to fuse aportion of a junction type electrode of a semiconductor unit with ametallic or eutectic connector body in such a way as to avoid thermaldisturbance of the semiconductor-electrode junction. Another specificobject is to accelerate the fusing process while maintaining highquality of the ultimate, fabricated semiconductor unit. Other specificobjects of the new process will appear from the description whichfollows.

In a preferred form of the new process, there is used a metal alloymelting and pure metal dissolving treatment, which is performed, rapidlyand yet precisely, by controlled, conductive heating of a metallicsupport ele ment for the metal alloy constituent. This will beunderstood upon a study of the following disclosure, wherein the newmethod will be explained in conjunction with the drawing appendedhereto.

FIGURE l of said drawing is a vertical, central sectaken on an enlargedscale and schematically showing a first sub-combination of elements,constituting the metal alloy carrier to be used in the new method.FIGURE 2a is a similar section, on a larger scale, showing a secondsubassembly of elements, which constitutes the central part of thesemiconductor device to be completed by this method. FIGURE 2b is asection similar to and on the scale of FIGURE 2a, indicating how thefirst and second subassem'blies are united with one another. FIG- URE 3is a graphic representation of certain thermal functions, forming partof the new method.

The method serves to unite an electrode connector and/or heat sinkmember 19, FIGURE 1, with a semiconductor unit such as the transistorsubassembly 20 of FIGURE 2a. The way in which these elements orsubcombinations are united, according to the present inven- 3,082,522Patented Mar. 26, 1963 tion, constitutes an improved modification of theinvention of C. G. Thornton, described in his application Serial No.590,204, filed June 8, 1956, now Patent No. 3,002,271, entitledFabrication Method and assigned to the assignee of this invention. TheThornton method used an immersion heating process, which constituted oneof the prior art techniques, initially mentioned. No such heating isused according to the new method.

In FIGURE 1, member 10 is shown in form of a metallic slug, having anintegral pedestal or boss '11 upstanding from a top surface 12, for thesupport of semiconductor subassembly 29 (FIGURE 2a). The lattersubrassembly is additionally provided, generally at a later time, withan electrode connection member 30 (*FIG- URE 2b), opposite slug 10.Still later, parts 10, 20 and 3%) must be electrically connected withsuitable circuitry.

For this latter purpose, a glass bead eylet 13 (FIG- URE 1) extends,from adjacent top surface 12, through slug 10, toward the opposite orbottom surface 14, and lead wires 15, 16, 17 extend through the glassbead. The upper ends of said wires are, respectively, connected (FIGURE2b) to certain parts of connection member 39, transistor subassembly 2t)and slug 10. A semiconductor blank 21, fonning the principal part ofsubassembly 20, is shown as being secured by a solder joint 26 to a tab27, this tab having a lug 28 for securement to Wire 16. Connections17-1ll and 2127 may be established prior to the operations involved inthe present method (see FIGURES 1 and 2a); the other connections aremade thereafter. in order to protect the device, a metallic closure orso-called hat, 40, may subsequently be coldwelded to the support slug10, in a flange region 41, FIGURE 1.

The transistor electrode subassembly 20 (FIGURE 20) comprises a. small,flat blank 21, made for instance of germanium or the like. A first phaseof the fabricating process involves the production of this subassemb-ly,that is, the provision of a bead-shaped emitter electrode member 22 inan upper central part of said blank and of a slightly larger, similarlyshaped collector electrode member 23 in a lower central part thereof.These operations may be carried out in any suitable ways, which need notbe described herein. The emitter electrode is preferably made of pureindium, with a controlled admixture of gallium, and the collectorelectrode is preferably made of pure indium without admixture.

As a result of this first phase of the process, the unit 20 comprisesregions 22, 23' of alloyed, recrystallized germanium and indium, orso-called P-type material, which regions are shown as extending fromelectrode members 22, 23 into the germanium body 21 and as being definedby boundaries or junctions, indicated by curved lines in FEGURES 2a, 2b.The said junctions are spaced apart by a flat and extremely thin layeror socalled base region 24 of unalloyed semiconductor material, orso-called N-type material, disposed within and parallel to the blank 21.In many cases the thickness of this base region 24 amounts only to asmall fraction of 21 mil.

It is a matter of great importance for the success of the device thatthe relation between the small regions or bodies 22 and 22', establishedin the production of the unit of FIGURE 2a, should not be disturbed andin fact not greatly modified in the process of securing workpiece 30 toelectrode 22. It is of equal importance that the similarly establishedrelation between the small bodies or regions 23, 23' should not bedisturbed in undesirable manner, although it must be modified to asubstantial extent, in the process of securing pedestal 11 of heat sink10 to the electrode 23. Heretofore the best practice, in both of thesephases of the fabrication process, involved the use of certain immersiontechniques, as particularly (FIGURE 2a).

taining high and satisfactory quality of the ultimate product.

A feature of importance in this connection has to do with the provisionof a secondary collector soldering element or bead 18 (FIGURE 1), whichdesirably consists of a cadmium-indium mixture and particularly of theeutectic of said materials. This soldering element is initially securedto a surface 18' on the boss 11, which surface may desirably be tinnedwith pure indium and which is positioned opposite the electrode member23 The beads 18, 23 are rounded so as to make it possible to initiallyestablish a small area 18" or so-called point of contact therebetween,and the mass and volume of the head 18 is so selected that upon thesubsequent melting thereof, it can dissolve all of the indium 23 and 18and no more.

In the process according to the present invention the eutectic 18 isbriefiy exposed to a temperature sufiicient to melt the same but only todissolve the indium 23 into the eutectic contacting it. The requiredheating is .to a temperature high enough to melt the eutectic 18 but nothigh enough to melt theindiurn 23, this narrow control being applied inorder to avoid disturbance of the alloy region 23'. As the eutectic 18melts and liquefies, which occurs as soon as the temperature thereofrises to the slightest extent above a very sharply defined melting andfreezing point, the liquid eutectic rapidly dissolves the indium 23 andbecomes indium-rich. The resulting liquid metal then flows along thesides of the boss 11,

. partly by virtue of the inherent adhesion of this liquid metal to theindium layer and partly by virtue of mechanical pressure which ismaintained on the contact area 18" and on the eutectic-indium interfacesexisting during the rapid, more or less momentary, melting anddissolving pIOCESS.

Thus the original solder member 18 and electrode member 23 are rapidlyconverted into a homogeneous and very thin electrode layer 23" ofindium-rich cadmiumind-ium alloy, which adheres both to the top surfaceof boss 11 and the bottom surface of blank 21, FIGURE 21). The ultimatethickness of this layer 23" is 'controlled by the cohesion of the liquidmetal therein, and the layer remains solid and imperforate under suchpressures as are applied thereto in the process according to thisinvention. The required, accurately parallel relationship between theblank 21 and the top of boss 11 is insured by suitable guidingmechanism, as described in the parent application. The excess ofenriched alloy, flowing along the indium tinned surface 18' of the boss11 and slug It forms a fillet 18' on said surfaces.

A feature of great importance for the present process is thatsubstantially all of the heat provided for melting the eutectic anddissolving the indium is supplied to these materials by conductionthrough the slug 10. It seems to be largely by virtue of this featurethat it is no longer necessary, as it was in the immersion processpreviously employed, to control the successive temperatures of allthermally coupled elements so as to insure a gradual tapering off of therate of heating up. It is, however, important that the rapid heating ofthe various parts, and particularly of the particles of eutectic, becontrolled, in a way which dilfers greatly from the control heretoforeapplied, for instance in accordance with the immersion heating method.In this connection the following should be noted.

A comparison between the new and the former process appears in FIGURE 3.In this figure the heat input into the indium, to be dissolved along theinterface 18" and the aforementioned subsequent interfaces, is shown atH while time is plotted along axis T. The broken line curve X isrepresentative of the manner in which an immersion process, for instancethat of said Thornton appli cation, supplied heat to the eutectic bodyand thereby caused the dissolving of the indium body. The heatingprocess in that case was gradual and gentle, as indicated by the slightand gradually decreasing inclination of the curve X from a horizontaldirection. At point A on the curve, sufiicient heat had been supplied tothe eutectic, along a variety of paths of heat transfer, to melt thefirst particles of the eutectic, which was promptly followed by thedissolving of the first particles of indium. Due to the provision of afinite although small mass of eutectic and of indium, and due to thegentleness of the heating process, all of the eutectic had beenliquefied only at point 'B on the curve. At that same point, orsubstantially so, all of the indium had been dissolved. On the aXis T,point B corresponded for instance to a time lapse of twenty or thirtyseconds after-the start of the process. Due to the gradual type ofheating employed, the temperatures of the pedestal were neversignificantly higher than those of the indium.

The full line curve Y shows, for comparison, the heat input applied tothe indium according to the present invention. It will be noted that theinitial rise of this curve is much steeper than that of curve X, andcorrespondingly the heat input into the pedestal is still more rapid, sothat it may briefly and locally establish extremely high temperatures,in the pedestal. If the heat input into the indium, curve Y, wereallowed to con tinue upwardly, as indicated at Z, it would ultimatelylevel off, in a manner similar to that of curve X, but this would happenonly after a heat input of such magnitude as to destroy the electrodemembers and associated parts. Actually, however, this heat input, asshown by the rise of the curve Y, is interrupted at or adjacent apredetermined point C, where it has not as yet created a temperature,anywhere in the pure indium, sufficient to melt this material. When thispoint C has been reached, the input of heat is interrupted and coolingof the small electrode assembly is initiated, as indicated by theturning and the subsequent falling of the curve Y.

Heat input values, corresponding to those shown at A and B on the curveX, areindicated at D and E on the curve Y, both of these points lyingbelow the point C and on the steeply rising and "substantially straightpart of the curve. It will be seen that the completion of the heatinput, which substantially coincides with the completion of thedissolving process, can be achieved in a time interval much shorter thanthat allowed in the immersion process. For instance, a heating period ofone or two seconds, or sometimes a small fraction of a second, has beenfound sufficient, in the use of the new method. The protection againstoverheating of indium, which in the case of curve X was obtained by thegradual decrease in the rise of the curve, is here obtained by theinterruption of the original, much steeper curve, at point C.

It has been found that substantially no loss, as to consistency ofproduction of satisfactory semiconductor assemblies, is incurred by thechange from the gradual immersion heating to the more rapid, suitablyinterrupted heating, effected entirely by conduction through the slugand pedestal. Particularly the new process has been found to besubstantially free from the danger that asymmetrical flow of metaloccurs at some points, such as that shown at F, in FIGURE 2b, Where someminute amounts of liquid metal 23" might flow beyond the exact boundaryof the recrystallized alloy zone 23. The danger of such minuteoverflowing of the solder-like material 2-3" is ever-present,particularly since it is not always possible to insure fully symmetricalfluxing of the pedestal area, to be wetted by the metal. If and whensuch overflowing occurs it can seriously impair the utility of thetransistor. The new process, in spite of the momentary use of hightemperatures, effectively avoids such overflowing and other dangers anddifliculties.

While only a single way of performing the new method has been described,it should be understood that the details thereof are not to be construedas limitative of the invention, except insofar as is consistent with thescope of the following claim.

I claim:

In the fabrication of a semiconductor: providing a semiconductor havinga junction type electrode formed thereon and including an external beadof the electrode metal; fusing to a relatively massive metallic contactmember a bead of eutectic of said electrode metal and of other materialso selected that said eutectic has a melting point lower than themelting points of the electrode metal,

metallic contact member, and semiconductor and that said eutectic whenmolten is a solvent for said electrode metal; placing the externalsurface of the electrode bead in contact with the external surface ofthe eutectic bead; and, while maintaining such contact, supplying heatto said eutectic by the step and exclusively by the step of rapidlyconducting intense heat into said metallic contact member and therebyinto said eutectic in an amount just suflicient to melt said eutecticbead, thereby to dissolve electrode metal of said external bead into themelting eutectic, and thus without substantial heating of said externalbead and electrode, closely thermally bonding said electrode to saidcontact member.

References Cited in the file of this patent UNITED STATES PATENTS1,695,791 Yunck Dec. 18, 1928 2,166,998 Morgan July 25, 1939 2,671,958Block Mar. 16, 1954 2,842,841 Schnable July 15, 1958 2,870,052 RittmannJan. 20, 1959 2,897,587 Schnable Aug. 4, 1959 2,947,079 Schnable Aug. 2,1960 2,985,806 McMahon et a1. May 23, 1961 3,002,271 Thornton Oct. 3,1961

